In many prior art nonaqueous dispersion polymerization systems, organic dispersing medium has been utilized having poor solvent properties for the polymer being produced. A dispersing agent was therefore utilized in the organic medium in order to disperse the polymer being formed throughout the medium. These dispersing agents or dispersants were generally polymeric materials such as block copolymers, random copolymers, or homopolymers as described in U.S. Pat. Nos. 4,098,980 and 4,452,960.
Styrene-butadiene rubbers (SBR) have generally been prepared in solvents in which SBR is soluble, however, only SBR's having a styrene content of less than 35% are soluble in hexane or other aliphatic solvents. These higher styrene content SBR polymers are not completely insoluble in the aliphatic solvents, and, in fact, are highly swollen in these solvents. However, SBR's having a styrene content greater than 35% necessarily have been polymerized in aromatic or cycloaliphatic solvents via solution polymerization.
The applicant first determined that certain pre-made dispersing agents can be utilized to conduct the nonaqueous dispersion polymerization production of SBR having a styrene content greater than 35% by weight in aliphatic dispersing medium such as hexane. In these first dispersion SBR studies, a single diblock polymer consisting of a short block (5-10% of total polymer) of hexane soluble polybutadiene and a long block (90-95% of total) of high styrene content SBR was prepared in the absence of a dispersing agent. Synthesis of this polymer structure in hexane resulted in either extremely viscous cements or the very undesirable phase separation. Although the dispersion process using a pre-made dispersant works well, it has one shortcoming from a practical or commercial scale up point of view. The dispersant must be prepared separately and stored for subsequent use in the polymerization process. Storage tank and transfer lines require a large capital expenditure and the synthesis of the dispersant and transfer time into the polymerization reactor results in higher production costs. It is therefore desirable to provide a dispersion polymerization process in which there is no need to store the dispersing agent prior to the commencement of the dispersion polymerization process.
It is an object of the present invention to provide a polymerization process to produce tapered copolymers of styrene and butadiene monomers in a nonaqueous dispersion into tapered copolymers having 30 to 70% by weight of vinyl aromatic hydrocarbon such as styrene in the presence of a dispersing agent prepared in situ, that is, during the polymerization reaction.
Such a nonaqueous dispersion polymerization process offers many advantages including improved stable dispersions, improved heat transfer, energy savings, high polymer concentrations in the reaction medium, increased production capacity, and the production of very high molecular weight tapered copolymers; and no need to store the dispersing agent prior to its use.
It is a further embodiment of the present invention that a stable dispersion of SBR can be made at styrene levels of 30 to 45% in the SBR in solvents of higher solubility using the in situ dispersion tapered copolymer process as compared to the in situ dispersion linear, random SBR copolymer process as shown in U.S. Pat. No. 5,331,035. Normally SBR having between a 30 to 35% styrene content is soluble in hexane. This new process extends the advantages of dispersion polymerization to a lower, unexpected styrene level.